1. Field of the Invention
This invention relates to an apparatus and a method for processing an image signal, and imaging equipment.
2. Description of Related Art
In the field of conventional movie production, film cameras have been used. This type of film camera has a mirror 81 mounted on a rotary disk shutter 80 off the open area of the shutter, and an imaging lens 70 and a ground glass 82 of an optical viewfinder aligned on the optical axis of the mirror 81 as shown in FIG. 1. When the rotary disk shutter 80 is positioned in the open area, photographed object images are formed on a film 83, exposing the film 83. Then this stops displaying the photographed object images on the optical viewfinder. When the rotary disk shutter 80 is positioned in the non-open area, frame stepping is carried out for the film 83, with the photographed object images reflected by the mirror 81 and displayed on the ground glass 82 of the optical viewfinder.
Regulation of the period of opening the shutter allows exposure time to be regulated. Changing the frame-stepping rate allows special effects to be obtained. For example, by replaying a high-speed motion picture (i.e. pictures taken at a high frame-stepping rate) at a normal speed, a high-speed motion of an object such as a water droplet dropping on the surface of water can be easily observed in detail. Replaying a low-speed picture (picture taken at a low frame-stepping rate) at a normal speed can enhance the reality of, for example, a fighting scene or a high-speed car chase.
In the field of TV program production, filming (or shooting), editing, and shipping of programs are in the process of digitalization. This is also the case in the field of movie production. That is, owing to the developments of digital techniques for extended definition pictures and to recent price-reduction of digital equipment, the digitalization is also promoted in the world of movie production.
When a video camera utilizing a solid-state image pickup device such as a charge coupled device (CCD) is used to shoot an object, control of shutter opening period of a film camera can be emulated by controlling exposure time using the electronic shutter function of the solid-state image pickup device. Also by controlling the read time required for retrieving a signal from the solid-state image pickup device, film-feeding speed of the film camera can be emulated. Further, since, unlike an optical viewfinder, an electronic viewfinder displays an image based on the signal retrieved from the solid-state image pickup device, the image can be displayed without any break.
Exposure time of a solid-state image pickup device is controlled using the electronic shutter function of the device as shown in FIG. 2A. Thus, a slow shooting (or low-speed shooting) can be obtained by prolonging the time intervals of images to be obtained, which can be done by extending the read time required for reading image data as shown in FIG. 2B. However, if a slow shooting is made in this way, images displayed on the electronic viewfinder are refreshed at a longer interval. As result, when panning the camera to a desired object in motion such that the image of the object is displayed at the center of the electronic viewfinder according to the displayed image on the electronic viewfinder if the object is fast one, the actual position of the image of the object on the imaged picture differs from the displayed position of the object in the viewfinder. Thus, the camera cannot correctly follow the shooting object.
On the other hand, when exposure time is extended while shooting a moving object, a resolution power for object in a time axis direction (hereinafter referred to as “dynamic resolution power”) decreases in the shot picture due to an increased movement of the object during one exposure time, making the displayed image blurred on the electronic viewfinder.
In a solid-state image pickup device, noise component called dark current is generated in photo-sensor for performing a photoelectric conversion. It is known that the dark current increases with exposure time. Hence, dark current increases when exposure time is increased to enhance the imaging signal. As a consequence, the margin of the dynamic range of the photo-sensor decreases with exposure time, as shown in FIG. 3. For example, when the object is bright, there are too many of imaging signals for processing to obtain an image having steady reproduction of halftones so as to cause a halation on a bright part thereof. If, in an attempt to increase the margin of the dynamic range, illumination is dimmed or an amount of light entering the solid-state image pickup device is reduced by decreasing the aperture of the object lens, a level of the dark current will not vary but only the intensity of the image signal decreases, thereby resulting in deterioration of the S/N ratio. Hence, a noiseless clear image cannot be obtained in this way.
In a CCD, noise components such as dark current and smear generated in the transmission lines for reading electric charges generated by the photoelectric conversion in the photo-sensor, increase with the read time required for reading signals. Hence, if the read time is increased, the margin of the dynamic range is further decreased according to the noise components, resulting in further deterioration of the S/N ratio.